How this online ductulator calculates duct size
The calculator checks a list of nominal round diameters or rectangular widths and returns the first size that meets both your friction-rate and velocity limits. Rectangular ducts use a fixed height and a maximum 4:1 aspect ratio. Friction is calculated for galvanized steel with standard air at approximately 70°F and sea level.
Pressure loss uses Darcy–Weisbach with a Swamee–Jain friction factor. Rectangular ducts use the Huebscher equal-friction diameter and its matching equivalent round velocity. Results cover straight duct friction only. Fittings, filters, coils, flex compression and available external static pressure require a separate system check.
Example: round duct for 400 CFM
With 400 CFM, a target friction rate of 0.08 in.wg/100 ft and a maximum velocity of 900 fpm, this catalogue selects a 12-inch round duct. Its calculated velocity is about 509 fpm and straight-duct friction is 0.035 in.wg/100 ft. The nearby 10-inch option exceeds the chosen friction limit. These limits are example inputs, not a system design recommendation.
Try this example in the calculatorFrequently asked questions
Is this ductulator free?
Yes. The online calculator is free to use without an account. The iPhone app offers a broader set of HVAC tools; some app features require a subscription.
Does it size flexible duct?
This web version sizes galvanized steel duct. Flexible duct has different roughness and installation losses, so do not use these friction results for flex duct.
What friction rate should I use?
Use a friction rate derived from available static pressure and the total effective length of your system. The starting value is an editable example, not a universal design recommendation.
Are all displayed sizes commercially available?
Nominal sizes are comparison options, not an inventory guarantee. Confirm local stock and fabrication sizes before specifying or ordering.